1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon processing. It particularly retates to the treatment of byproduct gases from plants for the production of detergent alkylate which utilize hydrogen fluoride as a catalyst.
2. Prior Art
Hydrogen fluoride catalysis finds one of its most important uses in the production of alkylated aromatic compounds. Of particular importance among alkylated aromatics are compounds referred to as detergent alkylate. These compounds are used in conjunction with others to synthesize biodegradable detergents, products of primary importance due to recent ecological considerations. Traditional, non-biodegradable detergents had come into such wide use that the characteristically low rate of their natural decomposition in the earth's surface waters was insufficient to prevent an accumulation of these chemicals in and a concomitant polution of the aqueous environment. Biodegradable detergents, derivatives of n-alkylbenzene, which have much higher rates of natural decomposition, have come into extensive use in order to avoid the contamination of surface waters.
Hydrofluoric acid is a hazardous chemical with properties peculiar to itself which call for special handling and treatment. With improper treatment it can be lethal. For this reason processes for its use must be equipped with systems which effectively prevent its escape into the atmosphere. Common practice in the art is to provide a relief system which collects the effluent of all relief valves and other sources within the process from which hydrogen fluoride may be expected to be released. As is well known in the art, relief valves are commonly fitted to processing zones which may operate at superatmospheric pressures. These valves open and allow an exhaust of material from the process at pressure levels above normal but below that at which structural damage to the processing zones would occur. It is quite common during the cessation or initiation of operation of a process that processing zones are periodically over-pressured. During these periods of overpressure, the associated relief valves open and maintain safe pressure levels by exhausting material from the affected zones. The exhausts from relief valves pass to a relief system which, in current plant designs, carries the exhaust to a treating process wherein the HF contained within the exhaust is chemically altered and made safe for entry into normal waste disposal facilities.
It is common in the art to use a treating process wherein acidic gases from the relief system are counter-currently contacted with an aqueous solution of a metal hydroxide, such as potassium hydroxide, within an elaborate plate-type contact tower. Where KOH is used as the metal hydroxide to treat a gas containing HF the ensuing reaction may be represented by the equation: EQU KOH + HF = KF + HOH.
the resulting aqueous KF solution is further contacted with Ca(OH).sub.2 to precipitate CaF.sub.2 which is highly insoluble in water. The fluoride precipitate, in the form of a sludge, is then disposed of as waste.
Operators of these prior art processes must replenish the HF lost by chemical treatment of waste gases. The disposal of a precipitate sludge also poses an inconvenience to the processor.
We have found that HF can be recovered from waste gases by the use of a hydrocarbon stream already existing within the process. In this manner HF leaving the process in waste gases is not chemically altered but is returned to the process for further use. HF loss from the process and the HF replenishment which loss necessitates are greatly reduced. The use of elaborate and inconvenient prior art processes involving chemical treatment and disposal of treatment wastes is avoided by use of the present invention.